Conventionally, a mobile-communication terminal has been realized by adding a wireless access interface to a portable terminal.
However, the wireless access networks accessed through such wireless access interfaces are different in coverage, bandwidth, communication fees, etc. depending upon the kind of interface. For this reason, in order to continue communication regardless of where the terminal is moved, there is a necessity to add the interfaces corresponding to a plurality of wireless access networks different in coverage or to exchange the interfaces from place to place.
Moreover, where there is a plurality of terminals, wireless access interfaces must be provided on each of the terminals. This however is problematic in respect of cost and size of the device.
To deal with this problem, there is disclosed in the document “Study on Seamless Handover on Mobile Network” (Y. Matsumoto et al., Shingakugiho, NS2002-323, March 2003) the following art. A group of mobile terminals are connected by means of short-distance wireless interfaces lower in cost and smaller in device size than the wireless access interface, one of the mobile terminals contains a wireless access interface and functions as a router, and the mobile terminals other than the router access an external network via the router. This art eliminates the necessity to mount the wireless access interfaces on all the mobile terminals.
Furthermore, for mobile communication, there is a need to switch between routers mounted with wireless access interfaces (e.g. cellular and wireless LAN) which are respectively different in coverage. The document “Study on Seamless Handover between Mobile Networks” discloses an art allowing continuation of communications regardless of place.
In this manner, in the IP (Internet Protocol), in order to allow communication between a terminal within a local area network (LAN) and a terminal outside the LAN, there is a need to set up a so-called default router in the apparatus of one terminal. The document “Neighbor Discovery for IP Version 6 (IPv6)” (T. Narten, et al., RFC2461, IETF, December 1998) discloses a neighbor discovery protocol (Neighbor Discovery for IP version 6, hereinafter referred to as ND) as a method to automatically set onto a terminal a default router chosen from the router apparatus.
In ND, the router transmits, at regular intervals, router advertisement messages (hereinafter, referred to as RA messages) containing the following pieces of information in response to a solicitation (router solicitation message, hereinafter referred to as RS messages) from the terminal:
(1) router IP address,
(2) valid term of the information contained in the message (router lifetime, hereinafter referred to as RL), and
(3) reachable time, the time when communication can be made (hereinafter referred to as RT).
The terminal, having received this information, adds the above pieces of information to its own default router list and neighbor cache.
The router selects a default router from among the entries in the list of default routers, and thus has communication with an external network.
Meanwhile, the router, when shutting itself down, transmits an RA with RL=0. When the terminal receives the RA message of RL=0 and if there is a corresponding entry in the default router list, the terminal deletes that entry.
Meanwhile, there is disclosed a method where in a double router apparatus, one of routers is made the master router and the other is made the backup router. A health check request from the backup router to the master router is made, and unless a health check response is received from the master router, the backup router is automatically switched to become the master router (see JP-A-11-261561, for example).
However, where using the method disclosed in the document “Neighbor Discovery for IP Version 6 (IPv6)”, an RA message with RL=0 is sent only upon shutting down of the default router by itself. As a result, the usual RA messages continue to be transmitted even when it is difficult or impossible to make connection to an external network due to an overload in the connection of the router to an external network or weakened electric-field strength. For this reason, in order to switch away from a default router that cannot make communication, the terminal is required to recognize that communication with the external terminal is impossible. However, this requires a great deal of time, leaving a problem of long interruptions in communication.
Also, in the method disclosed in JP-A-11-261561, the same MAC address must be shared among a plurality of routers, involving the problem that in some cases it cannot be used on a wireless network where authentication is made at the MAC layer.